Remove ThreadGlobalData

The only remaining client is ThreadLocalInspectorCounters,
but those are only ever read on the main thread. Instead
of storing them in thread-local data, we can just increment
and decrement them when on the main thread.

Review URL: https://codereview.chromium.org/26909005

git-svn-id: svn://svn.chromium.org/blink/trunk@159521 bbb929c8-8fbe-4397-9dbb-9b2b20218538